Non-contiguous channel allocation over multi-channel wireless networks

ABSTRACT

802.11ac networks define an operating band made of an ordered series of 20 MHz channels and authorize a restricted number of possible composite channel configurations to be used for data transmission. A method of transmitting data between a source and a receiver in such a wireless network may comprise, at the source: sending, to the receiver, RTS frames to request reservation of a composite channel, the RTS frames including a flag signalling the source supports transmission over un-authorized composite channels, for instance over sub-channels not contiguous within the operating band; receiving, from the receiver in response to the RTS frames, CTS frames acknowledging reservation of un-authorized composite channel configuration, for instance of non-contiguous sub-channels; and then transmitting, to the receiver, data frames on the reserved sub-channels of the un-authorized configuration, for instance on the reserved non-contiguous sub-channels within the operating band.

FIELD OF THE INVENTION

The present invention relates generally to communication networks andmore specifically to methods and devices for transmitting data over awireless communication network using Carrier Sense Multiple Access withCollision Avoidance (CSMA/CA), the network being accessible by aplurality of stations.

BACKGROUND OF THE INVENTION

Wireless local area networks (WLANs), such as a wireless medium in acommunication network using Carrier Sense Multiple Access with CollisionAvoidance (CSMA/CA), are founded on the principles of collisionavoidance. Such networks may also conform to a communication standardsuch as a communication protocol of 802.11 type e.g. Medium AccessControl (MAC).

The IEEE 802.11 MAC standard defines the way WLANs must work at thephysical and medium access control (MAC) level. Typically, the 802.11MAC (Medium Access Control) operating mode implements the well-knownDistributed Coordination Function (DCF) which relies on acontention-based mechanism based on the so-called “Carrier SenseMultiple Access with Collision Avoidance” (CSMA/CA) technique.

The 802.11 medium access protocol standard or operating mode is mainlydirected to the management of communication nodes waiting for the mediumto become idle so as to try to access to the medium.

The network operating mode defined by the IEEE 802.11ac standardprovides very high throughput (VHT) by, among other means, moving fromthe 2.4 GHz band which is deemed to be highly susceptible tointerference to the 5 GHz band, thereby allowing for wider frequencycontiguous channels of 80 MHz, two of which may optionally be combinedto get a 160 MHz channel as operating band of the wireless network. The802.11ac standard also tweaks the Request-To-Send (RTS) andClear-To-Send (CTS) frames to allow for composite channels of varyingand predefined bandwidths of 20, 40 or 80 MHz, the composite channelsbeing made of one or more contiguous sub-channels within the operatingband. The 160 MHz composite channel is possible by the combination oftwo 80 MHz composite channels within the 160 MHz operating band.

A composite channel therefore consists of a primary channel and at leasta secondary channel of for example 20 MHz each. The primary channel isused by the communication nodes to sense whether or not the channel isidle, which channel can thus be extended using the secondary channel toform a composite channel.

Tertiary and quaternary channels may also take part of the compositechannel.

A station is allowed to use as much channel capacity (or bandwidth, i.e.of sub-channels in the composite channel) as is available. Theconstraint is that the combined channels need to be contiguous for astation with a single antenna station (or single spatial stream).

However, if there is noise or interference on one of the 20 MHz channelwithin the wider composite channel, the available bandwidth is reduced.The 802.11ac standard only allows a restricted number of compositechannel configurations, i.e. of predefined subsets of 20 MHz channelthat can be reserved by the 802.11ac stations to transmit data. Theseare contiguous channels of 20, 40, 80 MHz bandwidth in case of singleantenna devices.

Therefore, noise or interference even on a small portion of thecomposite channel may substantially reduce the available bandwidth ofthe composite channel to only 40 or 20 MHz., since the resultingreserved bandwidth must meet the 20 MHz, 40 MHz, 80 MHz or 160 MHzchannel configurations allowed by the standard.

More recently, Institute of Electrical and Electronics Engineers (IEEE)officially approved the 802.11ax task group, as the successor of802.11ac. The primary goal of 802.11ax consists to improve data speedfor devices used in dense deployment scenarios.

The huge gigabit throughputs that are often attributed to 802.11ac aremainly theoretical. In fact, they represent the overall capacity a Wi-Finetwork can support, for instance 1.3 Gbps in today's most advancedrouters. However, they can occur only in the rarest circumstances whereany individual device would actually be able to connect at such highrates.

The existing 802.11ac standard requires that the composite channel widthbe specified in the 802.11ac frames, resulting in that channelsnon-contiguous within the operating band cannot be used although theyare available. Therefore, in the 802.11ax research context, there is aneed to enhance the efficiency and usage of the wireless channel.

Publication IEEE 802.11-13/1058r0 “Efficient Wider Bandwidth Operation”provided during the 802.11ax task group has raised the benefit of usingall available channels, even if no solution to do so has been provided.

SUMMARY OF INVENTION

It is a broad objective of the present invention to providecommunication methods and devices for transmitting data over an (ad-hoc)wireless network, the physical medium of which being shared between aplurality of communication nodes containing a single antenna device.

The present invention has been devised to overcome the foregoinglimitations.

In this context, the invention first provides a method of transmittingdata between a source node and a receiving node in an operating band ofa wireless network, the operating band being made of an orderedsuccession (or series) of sub-channels, the method comprising, at thesource node:

sending, to the receiving node, at least one medium access requestingframe to request reservation of a composite channel made of sub-channelsof the operating band, wherein the medium access requesting frameincludes a flag signalling the source node supports transmission overnon-contiguous sub-channels within the operating band, i.e. reservationof non-contiguous sub-channels;

receiving, from the receiving node in response to the medium accessrequesting frame, at least one medium access acknowledging frameacknowledging reservation of sub-channels belonging to the requestedcomposite channel that are not contiguous within the operating band,i.e. of sub-channels that form a reserved non-contiguous compositechannel;

transmitting, to the receiving node, data frames on the reservednon-contiguous sub-channels within the operating band.

From the receiving node's perspective, the invention also provides amethod of transmitting data between a source node and a receiving nodein an operating band of a wireless network, the operating band beingmade of an ordered succession of sub-channels, the method comprising, atthe receiving node:

receiving, from the source node, at least one medium access requestingframe to request reservation of a composite channel made of sub-channelsof the operating band;

determining an idle or busy status of each of the sub-channels formingthe composite channel, i.e. which requested sub-channels are availableand which ones are not;

determining whether or not the medium access requesting frame includes aflag signalling the source node supports transmission overnon-contiguous sub-channels within the operating band;

in case of positive determining, sending, to the source node and inresponse to the medium access requesting frame, at least one mediumaccess acknowledging frame acknowledging reservation of idlesub-channels belonging to the requested composite channel that are notcontiguous within the operating band, i.e. of idle or availablesub-channels that form a reserved non-contiguous composite channelwithin the operating band;

receiving, from the source node, data frames on the reservednon-contiguous sub-channels within the operating band.

Correlatively, the invention provides a source node for transmittingdata to a receiving node in an operating mode of a wireless network, theoperating band being made of an ordered succession of sub-channels, thesource node comprising at least one microprocessor configured forcarrying out the steps of the method defined above from the sourcenode's perspective.

Also the invention provides a receiving node for receiving data from asource node in an operating mode of a wireless network, the operatingband being made of an ordered succession of sub-channels, the receivingnode comprising at least one microprocessor configured for carrying outthe steps of the method defined above from the receiving node'sperspective.

The operating band may correspond to the band defined by the operatingmode of the wireless network. In the example of an 802.11ac operatingmode, the operating band may be an 80 MHz band or a 160 MHz band (madeof two frequency contiguous or non-contiguous 80 MHz sub-bands).Conventionally the operating band is made of successive sub-channelsordered according to their carrier frequencies. It means that thesub-channels of an operating mode are not necessary all frequencycontiguous (for instance in the case of the 80+80 MHz bandwidth).

The communication devices or nodes implementing the invention cancommunicate using sub-channels that are non-contiguous or “non-adjacent”or “non-successive” within the operating band (regardless of whether ornot the sub-channels are actually frequency contiguous), thus improvingnetwork efficiency.

This is achieved by indicating, in the conventional medium accessrequesting frame, such as an 802.11 Request-To-Send (RTS) frame, thatthe requesting node supports communication over non-contiguous channelswithin the operating band. Indeed, knowing this information, thereceiving node can efficiently adapt its response, namely by sending anacknowledgment of reservation for available sub-channels of therequested composite channel that are not contiguous within the operatingband, for instance by sending 802.11 Clear-To-Send (CTS) frames on eachof the sub-channels that are not contiguous within the operating band.

As a consequence, since those non-contiguous sub-channels within theoperating band have been correctly reserved, communication on thembetween the two source and receiving nodes may take place in a normalway.

This sharply contrasts with 802.11ac conventional behaviours where theCTS frames are only sent to acknowledge reservation of sub-channelswhich are contiguous within the operating band, as provided by the802.11ac fall-back schemes, thereby restricting the communicationbetween the source and receiving nodes to a narrower composite channel.

As it will be readily apparent from the description below, the sameapproach makes it possible to reserve and communicate over contiguoussub-channels within the operating band that do not meet the particularfall-back schemes as currently defined in 802.11ac or ax standard(namely 20 MHz, 40 MHz, 80 MHz channels that include the primarysub-channel). For instance, the signalling flag according to theinvention may be used to allow a 60 MHz composite channel to benegotiated between the source and receiving node, thereby enabling 60MHz communication between them to take place although it is forbidden in802.11ac.

Optional features of embodiments of the invention are defined in theappended claims. Some of these features are explained here below withreference to a method, while they can be transposed into system featuresdedicated to any device according to embodiments of the invention.

In embodiments, sending the medium access requesting frame includessending a duplicated request-to-send frame (in the meaning of theRTS/CTS handshake scheme according to 802.11ac standard) on eachsub-channel forming the composite channel. In other words, the sameframe including the above-described signalling flag is sent on eachsub-channel forming the requested composite channel; and

receiving the medium access acknowledging frame includes receiving aclear-to-send frame on each of the reserved non-contiguous sub-channelswithin the operating band.

From the receiving node's perspective, receiving the medium accessrequesting frame may include receiving a duplicated request-to-sendframe on each sub-channel forming the composite channel; and

sending the medium access acknowledging frame may include sending aclear-to-send frame on each of the reserved non-contiguous sub-channelswithin the operating band.

Thanks to this provision, the method according to the invention remainscompliant with 802.11ac standard regarding the reservation of acomposite channel, as well as compliant with any 802.11 standard (evenoldest ones) to ensure legacy nodes correctly understand the reservationof each sub-channel to avoid collisions.

In other embodiments, the medium access acknowledging frame identifies acontiguous composite channel within the operating band, which contiguouscomposite channel encompasses the reserved non-contiguous sub-channelswithin the operating band, and the medium access acknowledging frameincludes a flag signalling the receiving node supports transmission overnon-contiguous sub-channels within the operating band. Thanks to thisprovision, full compatibility with 802.11ac standard is kept. Inparticular, the medium access acknowledging frame (except the unknownsignalling flag) is normally processed by any 802.11ac node notimplementing the invention.

According to a particular feature, the medium access acknowledging framespecifies one of the channel widths defined in the 802.11ac channelbonding scheme, to identify the contiguous composite channel within theoperating band. Thanks to this provision, any legacy node receiving suchmedium access acknowledging frame will handle it in a conventional way,regardless of whether or not it is support of a transmission overchannels non-contiguous in the operating band. As defined in 802.11acstandard, such legacy node will enter a standby mode.

In particular, the specified channel width is preferably the narrowest802.11ac channel width that encompasses the reserved non-contiguoussub-channels within the operating band. This approach optimizes use ofthe network bandwidth.

In a variant, to identify the contiguous composite channel within theoperating band, the medium access acknowledging frame includes the samechannel width as a requested channel width included in the medium accessrequesting frame. Of course, such channel width is preferably (but notnecessarily) one of the channel widths defined in the 802.11ac channelbonding scheme.

In some embodiments, the composite channel defined in the medium accessrequesting frame is made of sub-channels that are non-contiguous withinthe operating band. This shows that the present invention may be usedeven when the source node detects available sub-channels that are notcontiguous within the operating band. This sharply contrasts with theconventional 802.11ac approach according to which the source node canonly request for one of the predefined bandwidths corresponding toavailable contiguous sub-channels within the operating band.

According to a particular feature, the medium access requesting frameidentifies a contiguous composite channel within the operating band,which contiguous composite channel encompasses the requested compositechannel. Thanks to this provision, full compatibility with 802.11acstandard is kept.

In particular, to identify the contiguous composite channel thatencompasses the requested composite channel, the medium accessrequesting frame specifies one of the channel widths defined in the802.11ac channel bonding scheme. This is to keep compliance with 802.11standard, in a way that any 802.11 node is kept informed regardless ofwhether it implements the invention or not.

According to embodiments, the signalling flag is included in a header ofthe medium access requesting frame and/or medium access acknowledgingframe. Thanks to this provision, the signalling flag can be easily addedto any 802.11ac frame. In addition, it can be easily made meaninglessfor 802.11 nodes not implementing the invention, ensuring backwardcompatibility.

For instance, the signalling flag includes one of the following bits:

bit 2 of VHT-SIG-A1 header portion according to the 802.11ac standard;

bit 23 of VHT-SIG-A1 header portion according to the 802.11ac standard;

bit 9 of VHT-SIG-A2 header portion according to the 802.11ac standard.

These specific bits are “reserved” bits in the 802.11ac standard. As aconsequence, using them as proposed in this above configuration does notimpact the behaviour of 802.11ac-compliant nodes that do not implementthe invention. This is because these bits are not taken into account bythese nodes. VHT-SIG-A1 and VHT-SIG-A2 form the two parts of the VHTSignal A field of the 802.11ac frame header, each of which correspondsto an OFDM symbol.

According to embodiments, the composite channel and the reservedsub-channels include a primary sub-channel according to the 802.11acstandard. Again, this provision contributes to keep full backwardcompatibility with any 802.11 standard.

According to embodiments, the data frames also include a flag signallingthe transmission between the source and receiving nodes is performedusing transmission over non-contiguous sub-channels within the operatingband. This signalling within the data frames makes it possible to signalthe current transmission mode (i.e. with or without non-contiguoussub-channels) to the receiving node. Thanks to this information, it isnot required for the latter to locally store and manage contextual datato keep knowledge of the specific current transmission mode over time.

Referring back to the exemplary use of the invention to enablecommunication on a 60 MHz composite channel made of three contiguoussub-channels as currently forbidden in 802.11ac, the present inventionalso provides a method of transmitting data between a source node and areceiving node in an operating band of a wireless network, the operatingband being made of an ordered succession of sub-channels, an operatingmode of the wireless network defining a restricted number of predefinedsub-channel subsets that are available for reservation by any wirelessnode of the wireless network to transmit data, the sub-channel subsetsbeing made of contiguous sub-channels within the operating band, themethod comprising, at the source node:

sending, to the receiving node, at least one medium access requestingframe to request reservation of a composite channel made of sub-channelsof the operating band, wherein the medium access requesting frameincludes a flag signalling the source node supports transmission oversubsets of sub-channels different from the predefined subsets;

receiving, from the receiving node in response to the medium accessrequesting frame, at least one medium access acknowledging frameacknowledging reservation of a subset of sub-channels different from thepredefined subsets;

transmitting, to the receiving node, data frames on the reserved subsetof sub-channels within the operating band.

From the receiving node's perspective, the invention also provides amethod of transmitting data between a source node and a receiving nodein an operating band of a wireless network, the operating band beingmade of an ordered succession of sub-channels, an operating mode of thewireless network defining a restricted number of predefined sub-channelsubsets that are available for reservation by any wireless node of thewireless network to transmit data, the sub-channel subsets being made ofcontiguous sub-channels within the operating band, the methodcomprising, at the receiving node:

receiving, from the source node, at least one medium access requestingframe to request reservation of a composite channel made of sub-channelsof the operating band;

determining an idle or busy status of each of the sub-channels formingthe composite channel;

determining whether or not the medium access requesting frame includes aflag signalling the source node supports transmission over subsets ofsub-channels different from the predefined subsets;

in case of positive determining, sending, to the source node and inresponse to the medium access requesting frame, at least one mediumaccess acknowledging frame acknowledging reservation of a subset ofsub-channels different from the predefined subsets;

receiving, from the source node, data frames on the reserved subset ofsub-channels within the operating band.

Correlatively, the invention provides a source node for transmittingdata to a receiving node in an operating mode of a wireless network, theoperating band being made of an ordered succession of sub-channels, anoperating mode of the wireless network defining a restricted number ofpredefined sub-channel subsets that are available for reservation by anywireless node of the wireless network to transmit data, the sub-channelsubsets being made of contiguous sub-channels within the operating band,the source node comprising at least one microprocessor configured forcarrying out the steps of the method defined above from the sourcenode's perspective.

Also the invention provides a receiving node for receiving data from asource node in an operating mode of a wireless network, the operatingband being made of an ordered succession of sub-channels, an operatingmode of the wireless network defining a restricted number of predefinedsub-channel subsets that are available for reservation by any wirelessnode of the wireless network to transmit data, the sub-channel subsetsbeing made of contiguous sub-channels within the operating band, thereceiving node comprising at least one microprocessor configured forcarrying out the steps of the method defined above from the receivingnode's perspective.

According to this approach of the invention, the signalling flag is usedto release or loose a constraint, such as the 802.11ac fallbackconstraint, of necessarily using one of the predefined contiguouscomposite channel configurations (i.e. one of the predefined subsets ofsub-channels that are available for reservation by any wireless node ofthe wireless network to transmit data). As a consequence, the receivingnode that implements the invention can accept using a non-predefinedconfiguration and thus send corresponding CTS frames (in 802.11),ensuring the other legacy nodes will not interfere on the reservedsub-channels.

As a result, a better use of the network bandwidth (operating band) isobtained). As an example, a 60 MHz composite channel may be obtained.Also un-authorized 40 MHz composite channels can be obtained (forinstance a primary sub-channel together with a tertiary or quaternarysub-channel, when the secondary sub-channel and one of the tertiary andquaternary sub-channels are busy).

The signalling flag to release the predefined configuration constraintcan be the same as the signalling flag introduced above to signal thesource node supports transmission over non-contiguous sub-channelswithin the operating band. In a variant, two separate flags may be used.

Optional features of embodiments of the invention are defined in theappended claims. Some of these features are explained here below withreference to a method, while they can be transposed into system featuresdedicated to any device according to embodiments of the invention.

In embodiments, the operating mode of the wireless network is accordingto the 802.11ac standard, and the restricted number of predefinedsub-channel subsets is made of 20 MHz, 40 MHz, 80 MHz and optionally 160MHz bandwidths within the operating band. These embodiments are specificto 802.11ac.

According to a specific feature, the reserved subset of sub-channels ismade of three 20 MHz sub-channels that are contiguous within theoperating band. Of course, as required by 802.11ac, the threesub-channels preferably include the “primary channel” on which thecontention process (i.e. backoff procedure) is performed by the nodesconcerned. Variants are directed to reserved subsets made of five, sixor seven 20 MHz contiguous sub-channels within the operating band.

In some embodiments, the reserved subset of sub-channels is made ofsub-channels that are not contiguous within the operating band. Forinstance, it may correspond to the use of non-contiguous sub-channels asdefined earlier in the document. As a consequence, all the embodimentsthat may derive from the use of a flag signalling the source nodesupports transmission over non-contiguous sub-channels within theoperating band as defined above can be used in the present embodiments.

In a variant, the reserved subset of sub-channels is made ofsub-channels that are contiguous within the operating band. Using 60 MHzcontiguous sub-channels is one example of this variant in the context of802.11ac.

Another aspect of the invention relates to a non-transitorycomputer-readable medium storing a program which, when executed by amicroprocessor or computer system in a device for transmitting databetween a source node and a receiving node in an operating band of awireless network, the operating band being made of an ordered successionof sub-channels, causes the device to perform any of the methods asdefined above.

The non-transitory computer-readable medium may have features andadvantages that are analogous to those set out above and below inrelation to the methods and devices.

Another aspect of the invention relates to a method of transmitting databetween a source node and a receiving node over a wireless network,substantially as herein described with reference to, and as shown in,FIG. 8, or FIGS. 8 and 12 of the accompanying drawings.

At least parts of the methods according to the invention may be computerimplemented. Accordingly, the present invention may take the form of anentirely hardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit”, “module” or “system”. Furthermore,the present invention may take the form of a computer program productembodied in any tangible medium of expression having computer usableprogram code embodied in the medium.

Since the present invention can be implemented in software, the presentinvention can be embodied as computer readable code for provision to aprogrammable apparatus on any suitable carrier medium. A tangiblecarrier medium may comprise a storage medium such as a floppy disk, aCD-ROM, a hard disk drive, a magnetic tape device or a solid statememory device and the like. A transient carrier medium may include asignal such as an electrical signal, an electronic signal, an opticalsignal, an acoustic signal, a magnetic signal or an electromagneticsignal, e.g. a microwave or RF signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention will become apparent tothose skilled in the art upon examination of the drawings and detaileddescription. Embodiments of the invention will now be described, by wayof example only, and with reference to the following drawings.

FIG. 1 illustrates a typical wireless communication system in whichembodiments of the invention may be implemented;

FIG. 2 is a timeline schematically illustrating a conventionalcommunication mechanism according to the IEEE 802.11 standard;

FIG. 3a illustrates 802.11ac channel allocation that support channelbandwidth of 20 MHz, 40 MHz, 80 MHz or 160 MHz as known in the art;

FIG. 3b illustrates an example of 802.11ac multichannel station using atransmission opportunity on an 80 MHz channel as known in the art;

FIG. 4 shows a conceptual diagram illustrating a broadband channelaccess mechanism employing an 80 MHz channel bandwidth as known in theart;

FIG. 5 illustrates three examples of dynamic fallback to narrowerchannel widths in the presence of co-channel interference or noise thatonly affects a portion of the larger channel;

FIG. 6 shows a schematic representation a communication device orstation in accordance with embodiments of the present invention;

FIG. 7 shows a schematic representation of a wireless communicationdevice in accordance with embodiments of the present invention;

FIG. 8 illustrates, using a flowchart, general steps of an enhancedchannel allocation method for multi-channel transmission to an 802.11acwireless medium, that allows usage of non-contiguous sub-channels, inaccordance with embodiments of the present invention;

FIGS. 9 and 10 illustrate exemplary communication timelines illustratingembodiments of the invention; and

FIGS. 11 and 12 illustrate a typical 802.11ac frame format as known inthe art, which is adapted to implement the invention according to someembodiments.

DETAILED DESCRIPTION

The invention will now be described by means of specific non-limitingexemplary embodiments and by reference to the figures.

FIG. 1 illustrates a communication system in which several communicationnodes exchange data frames over a radio transmission channel 100 of awireless local area network (WLAN). The radio transmission channel 100is defined by an operating band, for instance with a bandwidth availablefor the communication nodes.

Access to the shared radio medium to send data frames is based on theCSMA/CA technique, for sensing the carrier and avoiding collision byseparating concurrent transmissions in space and time.

Carrier sensing in CSMA/CA is performed by both physical and virtualmechanisms. Virtual carrier sensing is achieved by transmitting controlframes to reserve the medium prior to transmission of data frames.

Next, a transmitting node first attempts through the physical mechanism,to sense a medium that has been idle for at least one DIFS (standing forDCF InterFrame Spacing) time period, before transmitting data frames.

However, if it is sensed that the shared radio medium is busy during theDIFS period, the transmitting node continues to wait until the radiomedium becomes idle. To do so, it starts a countdown backoff counterdesigned to expire after a number of timeslots, chosen randomly between[0, CW], CW (integer) being referred to as the Contention Window. Thisbackoff mechanism or procedure is the basis of the collision avoidancemechanism that defers the transmission time for a random interval, thusreducing the probability of collisions on the shared channel. After thebackoff time period, the transmitting node may send data or controlframes if the medium is idle.

One problem of wireless data communications is that it is not possiblefor the transmitting node to listen while sending, thus preventing thetransmitting node from detecting data corruption due to channel fadingor interference or collision phenomena. A transmitting node remainsunaware of the corruption of the data frames sent and continues totransmit the frames unnecessarily, thus wasting access time.

The Collision Avoidance mechanism of CSMA/CA thus provides positiveacknowledgement (ACK) of the sent data frames by the receiving node ifthe frames are received with success, to notify the transmitting nodethat no corruption of the sent data frames occurred.

The ACK is transmitted at the end of reception of the data frame,immediately after a period of time called Short InterFrame Space (SIFS).

If the transmitting node does not receive the ACK within a specified ACKtimeout or detects the transmission of a different frame on the channel,it may infer data frame loss. In that case, it generally reschedules theframe transmission according to the above-mentioned backoff procedure.However, this can be seen as a bandwidth waste if only the ACK has beencorrupted but the data frames were correctly received by the receivingnode.

To improve the Collision Avoidance efficiency of CSMA/CA, a four-wayhandshaking mechanism is optionally implemented. One implementation isknown as the RTS/CTS exchange, defined in the 802.11 standard.

The RTS/CTS exchange consists in exchanging control frames to reservethe radio medium prior to transmitting data frames during a transmissionopportunity called TXOP in the 802.11 standard as described below, thusprotecting data transmissions from any further collisions.

FIG. 2 illustrates the behaviour of three groups of nodes during aconventional communication over a 20 MHz channel of the 802.11 medium:transmitting or source node 20, receiving or addressee or destinationnode 21 and other nodes 22 not involved in the current communication.

Upon starting the backoff process 270 prior to transmitting data, astation e.g. transmitting node 20, initializes its backoff time counterto a random value as explained above. The backoff time counter isdecremented once every time slot interval 260 for as long as the radiomedium is sensed idle (countdown starts from T0, 23 as shown in theFigure).

The time unit in the 802.11 standard is the slot time called ‘aSlotTime’parameter. This parameter is specified by the PHY (physical) layer (forexample, aSlotTime is equal to 9 μs for the 802.11n standard). Alldedicated space durations (e.g. backoff) add multiples of this time unitto the SIFS value.

The backoff time counter is ‘frozen’ or suspended when a transmission isdetected on the radio medium channel (countdown is stopped at T1, 24 forother nodes 22 having their backoff time counter decremented).

The countdown of the backoff time counter is resumed or reactivated whenthe radio medium is sensed idle anew, after a DIFS time period. This isthe case for the other nodes at T2, 25 as soon as the transmissionopportunity TXOP granted to transmitting node 20 ends and the DIFSperiod 28 elapses. DIFS 28 (DCF inter-frame space) thus defines theminimum waiting time for a transmitting node before trying to transmitsome data. In practice, DIFS=SIFS+2*aSlotTime.

When the backoff time counter reaches zero (26) at T1, the timerexpires, the corresponding node 20 requests access onto the medium inorder to be granted a TXOP, and the backoff time counter isreinitialized 29 using a new random backoff value.

In the example of the Figure implementing the RTS/CTS scheme, at T1, thetransmitting node 20 that wants to transmit data frames 230 sends aspecial short frame or message acting as a medium access request toreserve the radio medium, instead of the data frames themselves, justafter the channel has been sensed idle for a DIFS or after the backoffperiod as explained above.

The medium access request is known as a Request-To-Send (RTS) message orframe. The RTS frame generally includes the address of the receivingnode (“destination 21”) and the duration for which the radio medium isto be reserved for transmitting the control frames (RTS/CTS) and thedata frames 230.

Upon receiving the RTS frame and if the radio medium is sensed as beingidle, the receiving node 21 responds, after a SIFS time period 27 (forexample, SIFS is equal to 16 μs for the 802.11n standard), with a mediumaccess response, known as a Clear-To-Send (CTS) frame. The CTS frameindicates the remaining time required for transmitting the data frames,computed from the time point at which the CTS frame starts to be sent.

The CTS frame is considered by the transmitting node 20 as anacknowledgment of its request to reserve the shared radio medium for agiven time duration.

Thus, the transmitting node 20 expects to receive a CTS frame 220 fromthe destination node 21 before sending data 230 using unique and unicast(one source address and one addressee or destination address) frames.

The transmitting node 20 is thus allowed to send the data frames 230upon correctly receiving the CTS frame 220 and after a new SIFS timeperiod 27.

An ACK frame 240 is sent by the receiving node 21 after having correctlyreceived the data frames sent, after a new SIFS time period 27.

If the transmitting node 20 does not receive the ACK 240 within aspecified ACK Timeout (generally within the TXOP), or if it detects thetransmission of a different frame on the radio medium, it reschedulesthe frame transmission using the backoff procedure anew.

Since the RTS/CTS four-way handshaking mechanism 210/220 is optional inthe 802.11 standard, it is possible for the transmitting node 20 to senddata frames 230 immediately upon its backoff time counter reaching zero(i.e. at T1).

The requested time duration for transmission defined in the RTS and CTSframes defines the length of the granted transmission opportunity TXOP,and can be read by any listening node (“other nodes 22” in FIG. 2) inthe radio network.

To do so, each node has in memory a data structure known as the networkallocation vector or NAV to store the time duration for which it isknown that the medium will be busy. When listening to a control frame(RTS 210 or CTS 220) not addressed to itself, a listening node 22updates its NAVs (NAV 255 associated with RTS and NAV 250 associatedwith CTS) with the requested transmission time duration specified in thecontrol frame. The listening node 22 thus keeps in memory the timeduration for which the radio medium will remain busy.

Access to the radio medium for the other nodes 22 is consequentlydeferred 30 by suspending 31 their associated timer and then by laterresuming 32 the timer when the NAV has expired.

This prevents the listening nodes 22 from transmitting any data orcontrol frames during that period.

It is possible that the destination node 21 does not receive the RTSframe 210 correctly due to a message/frame collision or to fading. Evenif it does receive it, the destination node 21 may not always respondwith a CTS 220 because, for example, its NAV is set (i.e. another nodehas already reserved the medium). In any case, the transmitting node 20enters into a new backoff procedure.

The RTS/CTS four-way handshaking mechanism is very efficient in terms ofsystem performance, in particular with regard to large frames since itreduces the length of the messages involved in the contention process.

In detail, assuming perfect channel sensing by each communication node,collision may only occur when two (or more) frames are transmittedwithin the same time slot after a DIFS 28 (DCF inter-frame space) orwhen their own back-off counter has reached zero nearly at the same timeT1. If both transmitting nodes use the RTS/CTS mechanism, this collisioncan only occur for the RTS frames. Fortunately, such collision is earlydetected by the transmitting nodes since it is quickly determined thatno CTS response has been received.

As described above, the original IEEE 802.11 MAC always sends anacknowledgement (ACK) frame 240 after each data frame 230 received.

However, such collisions limit the optimal functioning of the radionetwork. As described above, simultaneous transmission attempts fromvarious wireless nodes lead to collisions. The 802.11 backoff procedurewas first introduced for the DCF mode as the basic solution forcollision avoidance, and further employed by the IEEE 802.11e standardto solve the problem of internal collisions between enhanced distributedchannel access functions (EDCAFs). In the emerging IEEE 802.11n/ac/axstandards, the backoff procedure is still used as the fundamentalapproach for supporting distributed access among mobile stations ornodes.

The rapid growth of smart mobile devices is driving mobile data usageand 802.11 WLAN proliferation, creating an ever-increasing demand forfaster wireless networks to support bandwidth-intensive applications,such as web browsing and video streaming. The new IEEE 802.11ac standardis designed to meet this demand, by providing major performanceimprovements over previous 802.11 generations.

The IEEE 802.11ac standard is an emerging very high throughput (VHT)wireless local access network (WLAN) standard that can achieve physicallayer (PHY) data rates of up to 7 Gbps for the 5 GHz band.

The scope of 802.11ac includes single link throughput supporting atleast 500 Mbps, multiple-station throughput of at least 1 Gbps andbackward compatibility and coexistence with legacy 802.11 devices in the5 GHz band.

Consequently, this standard is targeted at higher data rate servicessuch as high-definition television, wireless display (high-definitionmultimedia interface—HDMI—replacement), wireless docking (wirelessconnection with peripherals), and rapid sync-and-go (quickupload/download).

Contrary to 802.11n where each communication node should support up totwo spatial streams (SSs) and an operating band of 40 MHz bandwidth,only one spatial stream (and this only one antenna par device) isrequired in 802.11ac or 802.11ax, while operating bands of 80 MHz or 160MHz bandwidth are allowed.

One reason for such a change in the new versions of 802.11 is thatincreasing the number of antennas often results in higher cost. Indeed,supporting multiple Spatial Streams (SS) has been considered asrequiring at least the same number of antennas (and as much receptionchains behind these antennas), thus important costs. Consequently, amajority of 802.11n devices available on the market could only support asingle SS.

In 802.11ac, support for only one SS is required so that devices, andespecially smartphones, could be labelled as ‘802.11ac compliant’. Theoperating mode with an 80 MHz operating band is made mandatory as alower cost alternative to the two SS and 40 MHz operating band. Hence,the operating modes that utilize more than one spatial stream are nowbecome optional in 802.11ac.

As a result, 802.11ac is targeting larger bandwidth transmission throughmulti-channel operations. The wider channel aspect is further describedin regards to FIG. 3. A MAC mechanism for dynamically protecting andallocating multiple channels is presented in regards to FIG. 4.

In order to support wider channel bandwidths within the operating band,the operating band in 802.11ac is made of an ordered succession (orseries) of sub-channels as shown for instance in FIG. 3a . IEEE 802.11acintroduces support of a restricted number of predefined subsets of thesesub-channels to form the sole predefined composite channelconfigurations that are available for reservation by any 802.11ac nodeon the wireless network to transmit data.

The predefined subsets are shown in the Figure and correspond to 20 MHz,40 MHz, 80 MHz, and 160 MHz channel bandwidths, compared to only 20 MHzand 40 MHz supported by 802.11n. Indeed, the 20 MHz component channels(or sub-channels) 300-1 to 300-8 are concatenated to form widercommunication composite channels.

In the standard, the sub-channels of each predefined 40 MHz, 80 MHz or160 MHz subset are contiguous within the operating band, i.e. no hole(i.e. missing sub-channel) within the sub-channels as ordered in theoperating band is allowed.

The 160 MHz channel bandwidth is composed of two 80 MHz channels thatmay or may not be frequency contiguous. The 80 MHz and 40 MHz channelsare respectively composed of two frequency adjacent or contiguous 40 MHzand 20 MHz channels, respectively. The support of 40 MHz and 80 MHzchannel bandwidths is mandatory while support of 160 MHz and 80+80 MHzis optional (80+80 MHz means that a multi-channel is made of twofrequency non-contiguous channels having a bandwidth of 80 MHz).

A multichannel communication node (accessing an 80 MHz operating band inthe illustrated example of FIG. 3b ) is granted a TxOP through theenhanced distributed channel access (EDCA) mechanism on the “primarychannel” (300-3). Indeed, for each channel bandwidth, 802.11acdesignates one channel as “primary,” meaning that it is used fortransmission at that bandwidth. It shall, however, transmit an 80 MHzPPDU (PPDU means PLOP Protocol Data Unit, with PLOP for Physical LayerConvergence Procedure; basically a PPDU refers to an 802.11 physicalframe) only if all other channels have been idle for at least a pointcoordination function (PCF) inter-frame spacing (PIFS). If at least oneof the secondary channels has not been idle for a PIFS, then the nodemust either restart its backoff count, or use the obtained TxOP for 40MHz or 20 MHz PPDUs.

The vertical aggregation scheme reflects the extension of the payload230 to all sub-channels. If there is only one collision in one of thechannels at a given time, the risks of having a corrupted segment ofthese sequences are very high despite the error-correcting decodingprocess. All MPDU (MAC Protocol Data Unit) frames inside the PPDU couldthus automatically be considered as incorrect.

In the description below, the words “channel”, “20 MHz channel” or“sub-channel” mainly refer to the same technical feature, i.e. anychannel that complies with 802.11n or older standards. “Compositechannel” thus refers to the additional feature of 802.11ac in which acomposite channel is made of one or more sub-channels that arecontiguous within the operating band of the wireless network. In802.11ac, the composite channels are 20 MHz wide (if made of only onesub-channel) or 40 MHz wide or 80 MHz wide or, optionally, 160 MHz wide.

Further (FIG. 4), relying on its multi-channel capability, 802.11acsupports enhanced protection in which the RTS/CTS handshake mechanism ismodified to support static or dynamic bandwidth reservation and to carrythe channel bandwidth information.

Bandwidth signalling is added to the RTS and CTS frames (i.e. 20, 40, 80or 160 MHz values is added). A source node sends a RTS frame with anindication of the bandwidth of the intended transmission. The RTS frameis replicated on each 20 MHz channels forming the targeted bandwidth.The receiving node replies with a CTS frame on each (sub-)channel sensedas free.

As an example, prior to transmitting a 80 MHz data frame, the sourcenode, STA1, transmits an RTS frame 410 configured to use the 20 MHzchannel bandwidth of each of 20 MHz channels forming the 80 MHzoperating band. That is, in association with the 80 MHz operating band,a total of four RTS frames is transmitted in the form of duplicatedPPDUs over the four 20 MHz (sub-)channels.

The receiving node, STA2, answers in each 20 MHz channel in which an RTSframe transmitted by STA1 has been successfully received. The responseis made using a CTS frame configured to allow use of the respective 20MHz channel bandwidth. If STA2 has successfully received RTS frames fromthe entire 80 MHz bandwidth and the four (sub-)channels are sensed asidle, a total of four CTS frames is transmitted to cover the 80 MHzchannel bandwidth.

If STA1 receives all the four CTS frames related to the 80 MHz channel,a DATA frame 430-1 can be transmitted using the whole 80 MHz channelbandwidth.

It is expected that every nearby node (legacy or 802.11ac, i.e. which isneither STA1 nor STA2) can receive an RTS on its primary channel. Eachof these nodes then sets its NAV to the value specified in the RTSframe. Before a receiving node replies with a CTS, it checks if any ofthe channels in the 80 MHz band is busy. The receiving node only replieswith a CTS on those channels that are sensed as idle, and reports thetotal available bandwidth in the CTS. As with the RTS, the CTS is sentin an 802.11a PPDU format and is replicated over the different 20 MHzchannels that have been sensed as idle.

On the right side of FIG. 4, a nearby station or node is alreadytransmitting on the third 20 MHz channel as shown before STA2 startssending four RTS to reserve an 80 MHz TxOP. Next, STA1 has to informSTA2 by replying with CTSs only in the contiguous idle channels thatinclude the primary channel (two CTS are transmitted in the presentexample). Next, a DATA frame 430-2 can be transmitted using only 40 MHzchannel bandwidth. The ability of IEEE 802.11ac standard to fall back tolower bandwidth modes in case not all the targeted bandwidth isavailable is known as a fallback mechanism.

As addressed earlier, the IEEE 802.11ac standard enables up to four, oreven eight, 20 MHz channels to be bound. Because of the limited numberof channels (19 in the 5 GHz band in Europe), channel saturation becomesproblematic. Indeed, in densely populated areas, the 5 GHz band willsurely tend to saturate even with a 20 or 40 MHz bandwidth usage perWireless-LAN cell.

As a result, the fallback mechanism currently provided in the 802.11acstandard is too limitative.

A channel interference (FIG. 5) is typically performed by a legacy802.11a or 802.11n node (transmitting on a 20 MHz channel), so that the802.11ac node may transmit over a fraction of the original requestedbandwidth: depending of which 20 Mhz channels (300) are busy, thechannel width of resulting composite channel is reduced from requested80 MHz to a predefined channel width of the 802.11ac channel bondingscheme, namely to 40 MHz (cases 510 and 511) or to 20 Mhz (case 520),whereas a 60 Mhz (i.e. additional 20 MHz or 40 MHz respectively)bandwidth is potentially available.

One can note the 802.11ac standard has not envisaged using such lostbandwidth, as the RTS/CTS control frames embed a bandwidth indicationonly supporting predefined channel widths, namely 20, 40, 80 or 160 Mhz.

This bandwidth allocation deficiency can be especially problematic withpersonal devices on which a central organization has little or even nocontrol to select the wireless channels of the 5 GHz or the like band.This is ascertained in distributed environments, which are by essencenot managed at all.

The present invention falls within enhancement of the 802.11ac standard,and more precisely into the context of 802.11ax wherein dense wirelessenvironments are more ascertained to suffer from previous limitations.

The present invention provides enhanced channel allocation methods anddevices for data communication over an ad-hoc wireless network, thephysical medium of which being shared between a plurality ofcommunication stations (also referred to as nodes or devices).

An exemplary ad-hoc wireless network is an IEEE 802.11ac network (andupper versions) in which an operating band made of an ordered series ofsub-channels is used and in which a restricted number of predefinedcomposite channel configurations is available. However, the inventionapplies to any wireless network where a source node 101-107 sends dataof a data stream to a receiving node 101-107 using multiple channels(see FIG. 1). The invention is especially suitable for wireless stationshaving only one Spatial Stream and labeled as ‘802.11ac compliant’.

The behaviour of communication nodes during a conventional communicationover an 802.11 medium has been recalled above with reference to FIGS. 1to 5.

One aspect of embodiments of the present invention provides using mediumaccess requesting frames, i.e. RTS frames, that include a flagsignalling the source node supports transmission over sub-channels thatare not contiguous within the operating band. This makes it possible forthe receiving node to acknowledge reservation (using CTS frames) notonly of conventional contiguous sub-channels but also of idlesub-channels of the requested composite channel that are not contiguouswithin the operating band, i.e. of idle or available sub-channels thatform a reserved non-contiguous composite channel.

More generally, the signalling flag may signal the source node supportstransmission over subsets of sub-channels different from the predefinedsubsets, i.e. over non-authorized composite channel configurations.

In other words, the invention provides signalling means making itpossible to use non-contiguous channels within the operating band (andmore generally unauthorized composite channel configurations) inanticipation of the foreseen future evolution of the 802.11 standard.

One key aspect of the invention is that it still complies with legacynodes, i.e. with nodes that do not implement the invention. A legacyenvironment typically describes a situation in which the legacy nodesand the nodes implementing the invention coexist and are competing toaccess the shared wireless channels, possibly using a composite channel

The invention ensures the legacy nodes still operate in a conventionalway, despite some other nodes implement the invention. It means that thesignalling means according to embodiments of the invention areadvantageously transparent for those legacy nodes (i.e. not taken intoaccount).

FIG. 6 schematically illustrates a communication device 600 of the radionetwork 100, configured to implement at least one embodiment of thepresent invention. The communication device 600 may preferably be adevice such as a microcomputer, a workstation or a light portabledevice. The communication device 600 comprises a communication bus 613to which there are preferably connected:

-   -   a central processing unit 611, such as a microprocessor, denoted        CPU;    -   a read only memory 607, denoted ROM, for storing computer        programs for implementing the invention;    -   a random access memory 612, denoted RAM, for storing the        executable code of methods according to embodiments of the        invention as well as the registers adapted to record variables        and parameters necessary for implementing methods according to        embodiments of the invention; and    -   at least one communication interface 602 connected to the radio        communication network 100 over which digital data packets or        frames are transmitted, for example a wireless communication        network according to the 802.11ac protocol. The data frames and        aggregated frames are written from a FIFO sending memory in RAM        612 to the network interface for transmission or are read from        the network interface for reception and writing into a FIFO        receiving memory in RAM 612 under the control of a software        application running in the CPU 611.

Optionally, the communication device 600 may also include the followingcomponents:

-   -   a data storage means 604 such as a hard disk, for storing        computer programs for implementing methods according to one or        more embodiments of the invention;    -   a disk drive 605 for a disk 606, the disk drive being adapted to        read data from the disk 606 or to write data onto said disk;    -   a screen 609 for displaying decoded data and/or serving as a        graphical interface with the user, by means of a keyboard 610 or        any other pointing means.

The communication device 600 may be optionally connected to variousperipherals, such as for example a digital camera 608, each beingconnected to an input/output card (not shown) so as to supply data tothe communication device 600.

Preferably the communication bus provides communication andinteroperability between the various elements included in thecommunication device 600 or connected to it. The representation of thebus is not limiting and in particular the central processing unit isoperable to communicate instructions to any element of the communicationdevice 600 directly or by means of another element of the communicationdevice 600.

The disk 606 may optionally be replaced by any information medium suchas for example a compact disk (CD-ROM), rewritable or not, a ZIP disk, aUSB key or a memory card and, in general terms, by an informationstorage means that can be read by a microcomputer or by amicroprocessor, integrated or not into the apparatus, possibly removableand adapted to store one or more programs whose execution enables amethod according to the invention to be implemented.

The executable code may optionally be stored either in read only memory607, on the hard disk 604 or on a removable digital medium such as forexample a disk 606 as described previously. According to an optionalvariant, the executable code of the programs can be received by means ofthe communication network 603, via the interface 602, in order to bestored in one of the storage means of the communication device 600, suchas the hard disk 604, before being executed.

The central processing unit 611 is preferably adapted to control anddirect the execution of the instructions or portions of software code ofthe program or programs according to the invention, which instructionsare stored in one of the aforementioned storage means. On powering up,the program or programs that are stored in a non-volatile memory, forexample on the hard disk 604 or in the read only memory 607, aretransferred into the random access memory 612, which then contains theexecutable code of the program or programs, as well as registers forstoring the variables and parameters necessary for implementing theinvention.

In a preferred embodiment, the apparatus is a programmable apparatuswhich uses software to implement the invention. However, alternatively,the present invention may be implemented in hardware (for example, inthe form of an Application Specific Integrated Circuit or ASIC).

FIG. 7 is a block diagram schematically illustrating the architecture ofa communication device also called node (or station) 600 adapted tocarry out, at least partially, the invention. As illustrated, node 600comprises a physical (PHY) layer block 703, a MAC layer block 702, andan application layer block 701.

The PHY layer block 703 (here a 802.11 standardized PHY layer) has thetask of formatting and sending or receiving frames over the radio mediumused 100, such as 802.11 frames, for instance medium access requestingframes of the RTS type to reserve a transmission slot, medium accessacknowledging frames of the CTS type to acknowledge reservation of atransmission slot, as well as of MAC data frames and aggregated framesto/from that radio medium.

The MAC layer block or controller 702 preferably comprises a MAC 802.11layer 704 implementing conventional 802.11 MAC operations, and anadditional block 705 for carrying out, at least partially, theinvention. The MAC layer block 702 may optionally be implemented insoftware, which software is loaded into RAM 612 and executed by CPU 611.

Preferably, the additional block, referred to channel allocation module705, implements the part of the invention that regards node 600, i.e.transmitting operations for a source node, receiving operations for areceiving node and monitoring operations for any other node that is ableto understand the signalling flag according to the invention.

For both source node's perspective and receiving node's perspective,channel allocation module 705 senses which channels are available andcontributes to an enhanced dynamic channel allocation procedure even ifthe available channels (or “sub-channels”) form a non-contiguouscomposite channel or a composite channel that is different frompredefined composite channel configurations that are allowed, forinstance by a standard.

In this context, the source node is designed to build and send one ormore medium access requesting frame (e.g. RTS) that includes a flagsignalling the source node supports transmission over non-contiguoussub-channels within the operating band, i.e. reservation ofnon-contiguous sub-channels within the operating band.

On its side, the receiving node is designed to build and send (inresponse to RTS frame or frames) one or more medium access acknowledgingframe (e.g. CTS) to acknowledge reservation of non-contiguous idlesub-channels within the composite channel requested by the source node.

On top of the Figure, application layer block 701 runs an applicationthat generates and receives data packets, for example data packets of avideo stream. Application layer block 701 represents all the stacklayers above MAC layer according ISO standardization.

FIG. 8 illustrates, using a flowchart, general steps of an embodiment ofthe present invention to enhance channel allocation for multi-channeltransmission in an 802.11ac wireless medium.

FIGS. 9 and 10 illustrates exemplary communication lines according tothe invention. Although these examples show a WLAN system using amulti-channel including four contiguous sub-channels having a channelbandwidth of 20 MHz, the number of sub-channels or the channel bandwidththereof may vary.

Also these examples use the standard RTS/CTS handshaking mechanism overthe multiple channels according to 802.11ac. Of course, equivalentmechanisms may be used.

The method of FIG. 8 is implemented by two stations 600, one beingconsidered as a source node having data to transmit to the other node,namely a destination or receiving node.

At step 800, based on techniques such as described in 802.11ac standard,i.e. energy detection, preamble detection and/or plural channeldecoders, the source node is able to determine which of thecommunication 20 MHz sub-channels are idle for sending a correspondingRTS.

Let's consider the source node detects several sub-channels as idle andthus desires to reserve a composite channel L1 made of one or more ofthese available sub-channels.

At step 801, the source node sends a control or “medium accessrequesting” frame (RTS frame) to request reservation of the compositechannel made of detected free sub-channels 300-1 to 300-4 (see FIGS. 9and 10).

This frame is replicated on each 20 MHz sub-channel (300-i) forming thecomposite channel. According to the 802.11ac standard, the RTS frameincludes bandwidth information to indicate which 802.11ac channel widthis requested for data frame 830. This information is set in field 1200,called ‘BW’, of VHT-SIG-A1 portion of 802.11ac PLOP preamble accordingto 802.11ac protocol, as shown in FIGS. 11 and 12 discussed on below.

In the example of FIG. 9, all four sub-channels are sensed as idle andthus available for transmission. The source node thus duplicates the RTSframe intended for the primary (sub-)channel to the other threesub-channels. As a consequence, four RTS frames are sent, one on eachavailable 20 MHz channel.

In embodiments, the source node is able to apply an 802.11ac fallbackmechanism if one of the sub-channel is detected as busy, i.e. the sourcenode turns to request 40 MHz or 20 MHz, including the primary 20 MHzchannel.

In a variant, the source node doesn't apply the fallback scheme butemits the duplicated RTS in each idle sub-channel even if the idlesub-channels do not comply with a standardized contiguous 20 MHz or 40MHz or 80 MHz 802.11ac channel width. An example is shown in FIG. 10where the source node detects the tertiary channel as busy, meaning thatthe idle sub-channels form a non-contiguous composite channel within theoperating band: the source node determines that energy in the tertiarychannel is not below a CCA threshold, indicating this sub-channel isbusy or reserved by another communication node.

In this example of FIG. 10, the bandwidth indication 1200 in the RTSframe keeps the authorized value (40 MHz, 80 MHz or 160 MHz) of wholetargeted bandwidth, i.e. the contiguous bandwidth within the operatingband that encompasses the requested non-contiguous composite channel.This approach keeps backward compatibility with current 802.11standards.

According to the invention, the duplicated RTS frames includeinformation to direct the manner by which the source node is to providedata frames inside the granted TXOP, i.e. using only sub-channels thatare contiguous or adjacent within the operating band, or not. Suchinformation regards a capability indication of the source node tosupport non-contiguous channels within the operating band. As aconsequence, according to the invention, the medium access requestingframes (RTS frames) includes a flag signalling the source node supportstransmission over non-contiguous sub-channels within the operating band,i.e. reservation of non-contiguous sub-channels.

This information (signalling flag) may be located in a header portion ofthe RTS control frame, where the header portions otherwise conforms toIEEE 802.11ac standard. For example, the information may be included ina reserved field of a header portion (as for instance bits 1202, 1223 or1209 of the VHT PHY header as described in FIGS. 11 and 12 below).

This signalling flag will help the receiving node to optimize use ofavailable sub-channels from among the requested sub-channels.

Indeed, an 802.11ac-compliant node having one and a single antennacannot use channels that are not contiguous or successive within theoperating band, according to 802.11ac standard. Recently, papercontribution to 802.11ax have addressed the capability to communicateover a set of channels containing busy channels. The document IEEE802.11-15/0035r1, entitled “Scalable Channel Utilization Scheme”,proposes a scalable channel utilization scheme to utilize as manychannels as possible, even non-contiguous within the operating band, byturning on/off part of the OFDM tones. The OFDM Tones corresponding to adetected busy channel can be turned off so as to cause no interferencewith the ongoing transmission. For instance, the D(k) component of thefollowing expression (expressing the OFDM signal) can be set to zero:

${{d(n)} = {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}\;{{D(k)}{\exp\left( \frac{{j \cdot 2}{\pi \cdot k \cdot n}}{N} \right)}}}}},{n = 0},1,{{\ldots\mspace{14mu} N} - 1}$

where N is the FFT size.

Using the above-mentioned signalling flag, the present invention aims atsupporting such capability so that the nodes may discover thiscapability of talking in non-contiguous channels of the operating bandand may negotiate with each other a medium reservation includingsub-bands that are conventionally (i.e. according to 802.11ac) “notreachable” because not contiguous within the operating band.

The following of the process takes place if the receiving node (i.e. theaddressee as specified in the RTS frames) receives at least one of theRTS frames (test 802).

Upon receiving such RTS frame or frames, the receiving node determineswhich of the requested sub-channels (i.e. for which a RTS frame has beenreceived) are available (i.e. sensed as idle) using conventional sensingmeans. These available sub-channels form a list L2. A goal of suchdetermination is for the receiving node to be able to build or determinewhich composite channel is actually available for the source node, i.e.for which sub-channels forming this obtained composite channel thereceiving node has to send medium access requesting frames, for instanceCTS frames.

To do so, the receiving node, implementing the invention, needs to knowwhether or not the source device supports transmission overnon-contiguous sub-channels within the operating bans (in which casesub-channels that are not successive in the operating band could beactually reserved). This is done through step 804 in which the receivingnode analyses the received RTS frame or frames in order to retrieve andanalyse the value of the signalling flag as introduced above.

In case no signalling flag is set in the RTS frames (output “no” at test804), the conventional or “legacy” approach is applied through steps805-806. In case a signalling flag in the received RTS frames signalsthe source node supports transmission over non-contiguous channelswithin the operating band (output “yes” at step 804), an enhancedchannel reservation according to the invention may be implementedthrough steps 807-808.

If the receiving node determines that the source node doesn't supportnon-contiguous channel usage, the source node is considered as a legacyand the outcome of test 804 results in applying legacy allocation schemeat steps 805 and 806.

According to the 802.11ac standard, the list L2 determined by thereceiving node is based on the determination of the set of sub-channelsin which RTS control frames were received: list L2 can include secondarychannels 300-1,300-2,300-4 only if it also includes primary channel300-3 as in regards of FIG. 5.

In the examples of FIGS. 9 and 10, it means that quaternary channel300-1 will not be kept for reservation and considered for communicationbecause it is isolated due to the non-availability of tertiary channel300-2. Indeed, the combination of sub-channel 300-3, 300-4 and 300-1doesn't fit in an allowed 802.11ac bandwidth indication (20, 40, 80 or160 MHz). Thus, at step 805, the reserved composite channel L3 isdetermined from sub-channel list L2, wherein L3 contains the subset ofcontiguous sub-channels of L2 that meets a bandwidth allowed in 802.11acbonding scheme and that includes primary 20 MHz channel 300-3.

At step 806 in response to the RTS frames received, the receiving nodetransmits duplicated CTS response frames to the source node over eachsub-channel forming the determined contiguous composite channel L3. TheCTS frames indicate the resulting bandwidth according to 802.11ac legacyscheme. To be noted that, in order to keep backward compatibility (i.e.with 802.11a/b/g/n), the CTS frames follows the convention formatwithout any indication of the support of non-contiguous channels.

If the receiving node has successfully received all the RTS frames (forinstance the four RTS frames from the entire 80 MHz bandwidth shown inFIG. 9), a total of four CTS frames is transmitted to cover the 80 MHzchannel bandwidth. Therefore, according to 802.11ac standard, the whole80 MHz composite channel is reserved to the source node for it to sendits data to the receiving node.

If the receiving node has successfully received RTS frames only for apart of the requested bandwidth (for instance for a part of thesub-channels forming the 80 MHz bandwidth as shown in FIG. 9 becausetertiary channel 300-2 is busy in the standpoint of the receiving node),the receiving station will not transmit a CTS frame on quaternarychannel 300-1, but it will fallback to 802.11ac legacy scheme by issuingconventional CTS frames on 20 MHz channels 300-3 and 300-4 (40 MHzwidth). In other words, CTS frames are sent only for the compositechannel L3 corresponding to a bandwidth allowed in 802.11ac bondingscheme. As a consequence, the resulting bandwidth specified in the CTSframes is reduced compared to the original bandwidth BW specified in thereceived RTS frames.

Specific to the invention are steps 807 and 808 in case the receivingnode determines that a signalling flag in the received RTS framessignals the source node supports transmission over non-contiguouschannels within the operating band.

The determination step for reserved composite channel L3 is modifiedcompared to the legacy scheme described with reference to step 805. Atstep 807, the receiving node considers all available sub-channels listedin L2 (from among the original requested bandwidth) as potentialsub-channels for the further transmission.

Three main cases may occur regarding the available sub-channels of L2compared to the sub-channels of the requested composite channel L1:

-   -   in a first case, list L2 includes free sub-channels that are not        contiguous within the operating band;    -   in a second case, list L2 includes contiguous free sub-channels        within the operating band, which contiguous sub-channels do not        form an authorized channel width allowed by 802.11 channel        bonding scheme (i.e. do not correspond to predefined 20 MHz, 40        MHz or 80 MHz composite channel configurations that include the        primary sub-channel). This is for instance the case in example        520 of FIG. 5, wherein 60 MHz band is available but not        considered by 802.11ac standard;    -   in the third and last case, list L2 includes contiguous free        sub-channels that form an authorized channel width allowed by        802.11 channel bonding scheme. The resulting bandwidth is thus        compliant to the legacy scheme. This case also covers the        situation in which all requested sub-channels are free. This        case may be indifferently processed through the legacy        allocation scheme of steps 805 and 806 or through the enhanced        allocation scheme of steps 807 and 808.

At step 807, as the receiving node has detected non-contiguous freechannels (first case above) and determined that the source node doessupport their usage (i.e. transmission over non-contiguous channelswithin the operating band), it considers using all the non-contiguoussub-channels belonging to the requested composite channel of bandwidthBW.

The bandwidth indication, BW2, to be specified in the CTS response maybe restricted to the narrowest allowed 802.11ac channel width (to ensurebackward compatibility) that encompasses all these non-contiguoussub-channels. In a variant, it is kept identical to the bandwidth BWspecified in the received RTS frames.

Still at step 807 but for the second case, all free sub-channels arecontiguous within the operating band but do not comply with any channelwidth allowed by the 802.11ac standard (20, 40, 80 or 160 MHz). Thus,there are two possibilities.

First, the channel width, BW2, to be indicated in the CTS responses iskept as the original BW specified in the RTS frames received on thedetected free channels.

Second, BW2 is reduced to the 802.11ac bandwidth value (20, 40, 80 or160 MHz) which includes all the free contiguous sub-channels. As aresult, BW2 is less or equal than the requested bandwidth BW. In otherwords, BW2 is the 802.11ac standardized value just greater or equal tothe bandwidth formed by the detected free sub-channels.

In the example of FIG. 9 or 10, as BW equals 80 MHz and only onesub-channel (300-2) is busy among the four requested 20 MHz channels,BW2 also equals 80 MHz.

Still at step 807 but for the third and last case, all the detected freesub-channels are considered and the bandwidth they are forming is usedas BW2 to be specified in the CTS frames. Note that if all the requestedsub-channels are detected as idle, BW2 equals BW as requested by thesource node.

Next, at step 808, the receiving node generates a CTS frame thatincludes bandwidth BW2, and duplicates it on each channel of list L2(from which channels RTS frames were received). Furthermore, theduplicated CTS frames include an indication of the support ofnon-contiguous channels within the operating band, i.e. include the samesignalling flag as described above. This flag will help the source nodeto quickly know (as soon as the first CTS is received) whether or notthe receiving node supports transmission over non-contiguous channelswithin the operating band. This is because the source node may reduceits internal processing (for instance listening on various sub-channels,powering of antenna) in case the receiving node does not supportnon-contiguous channel usage.

Preferably, the CTS frames are in accordance to 802.11 standards toensure backward compatibility.

By receiving these CTS frames, the source node is allowed to configureitself for operating among bandwidth BW2 and knows if non-contiguouschannels are reserved. In the same way as done at step 803, the sourcenode can easily determine which channels are reserved as these channelscorrespond to the ones on which a CTS frame is received.

In the approach of the invention, if the receiving node has successfullyreceived RTS frames only from part of the 80 MHz bandwidth as shown inFIG. 9 or 10 (tertiary channel 300-2 being busy in the standpoint of thereceiving node), the receiving node can transmit less CTS frames (thanthe RTS frames sent) to cover the idle channels among the 80 MHz channelbandwidth. In the example of the Figure, there are three CTS frames thatare sent because the source node has indicated its capability oftransmitting over non-contiguous channels, and the receiving node isable to detect such capability, also to detect busy sub-channels (heretertiary channel 300-2), and to emit a CTS response replicated on eachof the detected idle sub-channels.

As a consequence, the source node is allowed to use a 60 MHz bandwidthchannel for instance in non-contiguous form (made of 830-1 and 830-2)for sending its data to the receiving node. This sharply contrasts tothe legacy approach in which only a 40 MHz transmission would beobtained by an 802.11ac-compliant source node.

Next to step 806 and 808, the CTS frames are received by the source nodeacting that a TXOP is granted to the source node. The latter is nowaware of the actual reserved bandwidth and starts transmitting data tothe receiving node sending DATA frames 830-1, 830-2 over the reservedcomposite channel. This is step 809.

In particular, the DATA frames can also include the signalling flag asdescribed above.

For a hardware point of view, transmitting data over channels notcontiguous within the operating band implied duplicating the front-endsegments per station for independent receive frequency filtering andsignal forming (front-end segments are composed of intermediatefrequency filtering and up/down conversion chains, including digital toanalog converter and analog to digital converter (ADC) respectively).

When using channels not contiguous within the operating band (first casementioned above), most used transmission systems comprise as manyfront-end segments as there are separate (i.e. non-contiguous) blocks ofsub-channels. It means that when the used sub-channels are allcontiguous, there is only need for one front-end segment; and when theused sub-channels are non-contiguous within the operating band, thereare at least two front-end segments.

Document IEEE 802.11-15/0035r1 “Scalable Channel Utilization Scheme”provides a scalable channel utilization scheme to communicate overnon-contiguous channels using a single antenna and a singlecorresponding front-end segment. The scalable channel utilization schemeutilizes as many channels as possible by turning on/off part of the OFDMtones among a band of channels. To be more precise, the D(k) componentof the following expression (expressing the OFDM signal) can be set tozero for each sub-channel “k” sensed as busy:

${{d(n)} = {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}\;{{D(k)}{\exp\left( \frac{{j \cdot 2}{\pi \cdot k \cdot n}}{N} \right)}}}}},{n = 0},1,{{\ldots\mspace{14mu} N} - 1}$

where N is the FFT size.

This technical approach makes it possible for the skilled man toimplement transmission over non-contiguous channels using only oneantenna.

The above has clearly shown that in a context of dense channel usagewhere the chances of finding an idle contiguous channel of 80 MHzbandwidth (according but not limited to the figure) are very limited,allowing the reservation of all free, even non-contiguous, requestedsub-channels according to the invention substantially improved usage ofthe available bandwidth.

To be noted the node's behaviour when legacy nodes are involved.

In particular, a 802.11 legacy node that is receiving one or more RTSframes (not addressed to it) determines the TXOP duration (included in aduration field in the MAC header of RTS frame for 802.11ac/n, orcomputed from rate and length indicated in the PHY header of RTS framefor older 802.11 standards) and waits until the end of this periodbefore performing clear channel assessment (CCA).

As consequence, the wireless medium is protected against access bylegacy nodes for at least the duration of data transmission 830.

Also, a receiving node that implements the invention may receive RTSframes from a legacy 802.11ac/n source node. In that case, the receivingnode of the invention may determine that the RTS frames doesn't containthe above-mentioned signalling flag (or more generally a capabilityindication of supporting non-contiguous channels in the operating band)and then consider the source node as a legacy node.

To ensure backward compatibility of the RTS, CTS and DATA frames witholder 802.11 standards (i.e. to ensure the old legacy nodes are able toread and process these frames), the signalling flag is preferably set ina part of the frames that does not impact the legacy nodes that doimplement the invention.

For instance, the signalling flag is included in a header of the RTS orCTS or DATA frames.

It is now described with reference to FIGS. 11 and 12 which showpossible locations for the signalling “non-contiguous channel usage”flag inside an 802.11 frame, in particular using one of the followingbits:

bit 2 of VHT-SIG-A1 header portion according to 802.11ac standard;

bit 23 of VHT-SIG-A1 header portion according to 802.11ac standard;

bit 9 of VHT-SIG-A2 header portion according to 802.11ac standard.

The 802.11ac frame format is shown in FIG. 11 and starts, as expected,with a preamble or header.

The first three fields are L-STF (Short Training Field), L-LTF (LongTraining Field) and L-SIG (Signal) well known by one skilled in the artwhen considering 802.11ac standard.

The L-STF and L-LTF fields contain information that allows the device todetect the wireless signal, perform frequency offset estimation, timingsynchronization, etc. The ‘L-’ stands for ‘legacy’ and the details ofthe sequences used in these fields for the 20 MHz signals are the sameas the legacy 802.11a and 802.11n preamble fields which allows for all802.11 devices to synchronize with the wireless signal.

In addition, the L-SIG field includes information regarding the lengthof the rest of the frame. This means that all devices including thelegacy devices will know that a frame of a given length is beingtransmitted.

The next fields in the frame, the names of which start with “VHT”, arespecific to 802.11ac (“VHT” means 802.11ac since it stands for “VeryHigh Throughput”). The VHT-SIG-A field contains two OFDM symbols, namelyVHT-SIG-A1 and VHT-SIG-A2.

The first symbol is modulated using BPSK, so that any 802.11n devicereceiving it will think that the frame is in accordance with the 802.11aframe format.

Important information is contained in the bits of these two symbols suchas bandwidth mode, MCS (Modulation and Coding Scheme) for the singleuser case, number of space time streams, etc.

The legacy fields and the two VHT-SIG-A fields are duplicated over each20 MHz of the bandwidth when 802.11ac (including the invention) isimplemented.

After the VHT-SIG-A section, the VHT-STF field is sent. The primaryfunction of the VHT-STF field is to improve automatic gain controlestimation in a MIMO transmission.

The next 1 to 8 fields of the packet are the VHT-LTF fields, one perspatial stream (up to SS=8 usually) to be used for transmission. VHT-LTFfields allow the receiving node to calculate the multipathcharacteristics of the channel and apply them to the MIMO algorithm.

The VHT-SIG-B field is the last field in the preamble/header before thedata field is sent. VHT-SIG-B is BPSK modulated and provides informationon the length of the useful data in the packet and in the case ofMU-MIMO provides the MCS (The MCS for single user case is transmitted inVHT-SIG-A).

Following the preamble/header, data symbols are transmitted.

FIG. 12 shows the VHT-SIG-A section for the single user case with thenumber of bits used for each of its fields.

The bandwidth field “BW” is made of two bits used to indicate thecomposite channel bandwidth: 0 for 20 MHz, 1 for 40 MHz, 2 for 80 MHz,and 3 for 160 MHz.

For single-user frame transmission, a partial association ID (partialAID) field is an abbreviated indication of the intended addressee of theframe, which thus enables any receiving node to enter power save modewhen it ascertains that it is not the intended recipient. Fortransmissions to an AP (access point), the partial AID is the last ninebits of the BSSID. For a receiving node, the partial AID is anidentifier that combines the association ID and the BSSID of its servingAP.

A Group identifier (group ID) field was introduced in the VHT-SIG-Afield. The downlink MU-MIMO transmissions can be organized in the formof MU-TXOP to facilitate the sharing of TXOP where AP can performsimultaneous transmissions to multiple receiving nodes by using thegroup ID. Nodes can determine whether they are part of the multiple-usertransmission or not.

According to the invention, there is a need to include the signallingflag, i.e. an indication of using non-contiguous channels for a currenttransmission (both in RTS/CTS frame handshake and DATA transmission)inside the 802.11 header frame. As the 802.11ac transmits an indicationof the bandwidth in the Bandwidth field, embodiments provides that thesignalling flag is included in a reserved field of the VHT-SIG-A headerof the 802.11 frame.

As a result, legacy 802.11a and 802.11n nodes will not decode suchindication.

In addition, if located in a reserved field, a legacy 802.11ac node maynot consider the non-contiguous channels signalling flag and ignore thiscapability.

Preferably, bit 2 of VHT-SIG-A1 is used (1202 in regards to FIG. 12)because it is the closest free bit to the Bandwidth field BW. But, otherlocations, such as for example 1223 (bit 23 of VHT-SIG-A1) and 1209 (bit9 of VHT-SIG-A2) are possible variants.

Although the present invention has been described hereinabove withreference to specific embodiments, the present invention is not limitedto the specific embodiments, and modifications will be apparent to askilled person in the art which lie within the scope of the presentinvention.

Many further modifications and variations will suggest themselves tothose versed in the art upon making reference to the foregoingillustrative embodiments, which are given by way of example only andwhich are not intended to limit the scope of the invention, that beingdetermined solely by the appended claims. In particular the differentfeatures from different embodiments may be interchanged, whereappropriate.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that different features are recited in mutuallydifferent dependent claims does not indicate that a combination of thesefeatures cannot be advantageously used.

The invention claimed is:
 1. A method of transmitting data between afirst node and a second node in a wireless network, the methodcomprising: sending, from the first node to the second node, a firstsignal which indicates a plurality of sub-channels that the first nodesupports; receiving, at the first node from the second node in responseto the first signal, a second signal which comprises information aboutnon-contiguous sub-channels over which the second node is ready fortransmitting data from among the plurality of sub-channels that thefirst node supports; and communicating, between the first and secondnodes, data over sub-channels among the non-contiguous sub-channelsreferred to in the information comprised in the second signal, whereinthe first signal identifies a contiguous composite channel made ofsub-channels, which contiguous composite channel encompasses thenon-contiguous sub-channels referred to in the information comprised inthe second signal.
 2. The method of claim 1, wherein sending the firstsignal includes sending a duplicated first signal on each sub-channel ofthe plurality of sub-channels.
 3. The method of claim 1, wherein theplurality of sub-channels indicated in the first signal is made ofsub-channels that are non-contiguous within an operating band of thewireless network.
 4. The method of claim 1, wherein the first signalidentifies a contiguous composite channel within an operating band ofthe wireless network, which contiguous composite channel encompasses theplurality of sub-channels indicated in the first signal.
 5. The methodof claim 1, wherein the indication of the plurality of sub-channels thatthe first node supports is included in a header of the first signal. 6.The method of claim 1, wherein both the plurality of sub-channels thatthe first node supports and the set of non-contiguous sub-channels overwhich the second node is ready for transmitting data include a primarysub-channel according to the 802.11ac standard.
 7. The method of claim1, wherein the plurality of sub-channels belongs to an operating band ofthe wireless network, the operating band being made of contiguousfrequency sub-channels.
 8. The method of claim 1, wherein the pluralityof sub-channels belongs to an operating band of the wireless network,the operating band being made of non-contiguous frequency sub-bands. 9.A method of transmitting data between a first node and a second node ina wireless network, the method comprising: receiving, at the second nodefrom the first node, a first signal which indicates a plurality ofsub-channels that the first node supports; determining an availabilitystatus of sub-channels that the first node supports; sending, from thesecond node to the first node in response to the first signal, a secondsignal which comprises information about non-contiguous sub-channelsthat are determined as being available for the second node; andcommunicating, between the first and second nodes, data oversub-channels among the non-contiguous sub-channels referred to in theinformation comprised in the second signal, wherein the first signalidentifies a contiguous composite channel made of sub-channels, whichcontiguous composite channel encompasses the non-contiguous sub-channelsreferred to in the information comprised in the second signal.
 10. Themethod of claim 9, wherein receiving the first signal includes receivinga duplicated first signal on each sub-channel of the plurality ofsub-channels.
 11. The method of claim 9, wherein the plurality ofsub-channels indicated in the first signal is made of sub-channels thatare non-contiguous within an operating band of the wireless network. 12.The method of claim 9, wherein the first signal identifies a contiguouscomposite channel within an operating band of the wireless network,which contiguous composite channel encompasses the plurality ofsub-channels indicated in the first signal.
 13. The method of claim 9,wherein the information about non-contiguous sub-channels over which thesecond node is ready for transmitting data is included in a header ofthe second signal.
 14. The method of claim 9, wherein both the pluralityof sub-channels that the first node supports and the set ofnon-contiguous sub-channels over which the second node is ready fortransmitting data include a primary sub-channel according to the802.11ac standard.
 15. The method of claim 9, wherein the plurality ofsub-channels belongs to an operating band of the wireless network, theoperating band being made of contiguous frequency sub-channels.
 16. Themethod of claim 9, wherein the plurality of sub-channels belongs to anoperating band of the wireless network, the operating band being made ofnon-contiguous frequency sub-bands.
 17. A first node for transmittingdata to a second node in a wireless network, the first node comprisingat least one microprocessor configured for carrying out the steps of:sending, to the second node, a first signal which indicates a pluralityof sub-channels that the first node supports; receiving, from the secondnode in response to the first signal, a second signal which comprisesinformation about non-contiguous sub-channels over which the second nodeis ready for transmitting data from among the plurality of sub-channelsthat the first node supports; and exchanging, with the second node, dataover sub-channels among the non-contiguous sub-channels referred to inthe information comprised in the second signal, wherein the first signalidentifies a contiguous composite channel made of sub-channels, whichcontiguous composite channel encompasses the non-contiguous sub-channelsreferred to in the information comprised in the second signal.
 18. Asecond node for receiving data from a first node in a wireless network,the second node comprising at least one microprocessor configured forcarrying out the steps of: receiving, from the first node, a firstsignal which indicates a plurality of sub-channels that the first nodesupports; determining an availability status of sub-channels that thefirst node supports; sending, to the first node in response to the firstsignal, a second signal which comprises information about non-contiguoussub-channels that are determined as being available for the second node;and exchanging, with the first node, data over sub-channels among thenon-contiguous sub-channels referred to in the information comprised inthe second signal, wherein the first signal identifies a contiguouscomposite channel made of sub-channels, which contiguous compositechannel encompasses the non-contiguous sub-channels referred to in theinformation comprised in the second signal.